With the development of digital technology, in recent years there is a growing tendency to process or store large-capacity data at high speed. Accordingly, demands have been made that the integration level and performance of semiconductor devices used in electronic apparatus be increased.
To increase the integration level of semiconductor memories such as dynamic random access memories (DRAMs), investigations on a technique for using a ferroelectric material or a high-dielectric-constant material in place of a silicon oxide (SiO) film or a silicon nitride (SiN) film which has traditionally been used for forming a capacitor insulating film are begun.
Flash memories and ferroelectric random access memories (FeRAMs) have traditionally been known as nonvolatile memories in which stored information is not lost at the time of turning off power.
A flash memory has a structure in which a floating gate is embedded in a gate insulating film of an insulated gate field-effect transistor (IGFET). Information is stored by accumulating electric charges on the floating gate. To write/erase information, it is necessary to pass a tunnel current through the insulating film. Accordingly, a comparatively high voltage is applied.
A FeRAM includes a ferroelectric capacitor in which a ferroelectric film used as a capacitor insulating film is between a pair of electrodes. Polarization is caused in the ferroelectric capacitor according to voltage applied between the electrodes. After the applied voltage is eliminated, spontaneous polarization is caused in the ferroelectric capacitor. Inversion of the polarity of the applied voltage causes inversion of the polarity of the spontaneous polarization. This spontaneous polarization is detected and information is read out. Compared with a flash memory, a FeRAM operates at low voltage and enables low-power high-speed writing. The use of a system on chip (SOC) in which a FeRAM is adopted in, for example, an IC card is being examined.
A Bi layer structure compound such as lead zirconate titanate (PZT) or strontium bismuth tantalate (SrBi2Ta2O9) is used as a ferroelectric film in a FeRAM. A sol-gel method, a sputtering method, a metal organic chemical vapor deposition (MOCVD) method, or the like is used for forming the ferroelectric film. If the sol-gel method or the sputtering method is used for forming the ferroelectric film, usually an amorphous or microcrystalline ferroelectric film is formed first over a lower electrode. Heat treatment is then performed for changing the crystal structure of the ferroelectric film to a perovskite structure or a Bi layer structure. If the MOCVD method is used for forming the ferroelectric film, the ferroelectric film is formed at a high temperature. Accordingly, a crystal structure such as a perovskite structure or a Bi layer structure is obtained in this formation process.
When an electrode of a ferroelectric capacitor is formed, it is necessary to use a material which is hard to oxidize or which can maintain conductivity even after oxidation. Usually a platinum metal, such as platinum (Pt) or iridium (Ir), or an oxide of a platinum metal, such as iridium oxide (IrOX), is used.
Usually aluminum (Al) is used as a material for forming a wiring of a FeRAM. This is the same with ordinary semiconductor devices.
Demands have been made that the integration level and performance of FeRAMs be increased. This is the same with the other semiconductor devices. Accordingly, it is necessary in the future to reduce the area of cells. The structures of FeRAMs are divided broadly into two categories: a planar structure and a stack structure. However, to reduce the area of cells, it is effective to adopt the stack structure. With a FeRAM having the stack structure, a barrier metal, a lower electrode, a ferroelectric film, and an upper electrode are formed in that order directly over a plug connected to a source/drain region of a transistor to form a ferroelectric capacitor. The barrier metal has the function of preventing the plug from being oxidized. In recent years lower electrodes often have the function of a barrier metal, so it is impossible to clearly differentiate lower electrodes from barrier metals. However, a material such as titanium nitride (TiN), aluminum titanium nitride (TiAlN), Ir, IrO2, Pt, or strontium ruthenate (SrRuO3 (SRO)) is used for forming a member corresponding to the barrier metal and the lower electrode.
To manufacture a FeRAM which has good electrical properties and the product yield of which is high, it is important to exercise control so as to make the orientation of a ferroelectric film included in a ferroelectric capacitor as uniform as possible. The orientation of the ferroelectric film is greatly influenced by the orientation of a lower electrode formed thereunder. Therefore, by exercising control so as to make the orientation of the lower electrode as uniform as possible, it is possible to improve the orientation of the ferroelectric film formed thereover.
In the past, a method for using, for example, an IrO2 (30 nm)-Ir (30 nm)-Ti (30 nm)-TiN (50 nm) laminated structure for a member corresponding to a lower electrode and a barrier metal (see Japanese Laid-open Patent Publication No. 2005-159165) or a method for using a Pt—PtOX—IrOX—Ir laminated structure for a lower electrode for the purpose of decreasing a leakage current passing through a ferroelectric capacitor (see Japanese Laid-open Patent Publication No. 2003-197874) was proposed.
Furthermore, a method for forming an Ir film at a temperature of 400° C. to 550° C. and an IrOX film at a temperature of 530° C. to 550° C. by the sputtering method at the time of forming a lower electrode by the use of Ir and IrOX (see Japanese Laid-open Patent Publication No. 2001-237392) or a method for using an IrO2 film in which IrO2/Ir is greater than or equal to 10 from X-ray diffraction intensity at the time of forming a lower electrode by the use of Ir and IrOX (see Japanese Laid-open Patent Publication No. 2002-151656) was proposed. In addition, a method for forming IrO2—Ir laminated structure continuously by the sputtering method (see Japanese Laid-open Patent Publication No. 2000-91270) or a method for forming an Ir film at a temperature of 450° C. or higher, forming an IrO2 film at a temperature of 300° C. or higher, and performing heat treatment at a temperature of 350° C. or higher for the purpose of improving the orientation of a PZT film (see Japanese Laid-open Patent Publication No. 2003-68991) was proposed.
Moreover, for example, (1 a method of forming an IrO2+ film over an Ir film, (2) a method for forming a crystalline IrO2 film over an Ir film, forming an amorphous IrO2 film over the crystalline IrO2 film, reducing the amorphous IrO2 film at the time of forming a PZT film by the MOCVD method, and oxidizing the amorphous IrO2 film again, (3) a method for forming an oxygen-doped Ir film over an Ir film, or (4) a method for forming an IrO2 film over an Ir film, forming an Ir film over the IrO2 film, and diffusing oxygen into the Ir film at the time of forming a PZT film by the MOCVD method was proposed (see Japanese Laid-open Patent Publication No. 2003-282844, U.S. Pat. No. 6,500,678, U.S. Pat. No. 6,528,328, U.S. Pat. No. 6,548,343, U.S. Pat. No. 6,596,547, U.S. Pat. No. 6,635,497, U.S. Pat. No. 6,686,236, and U.S. Pat. No. 6,872,669).
In addition, a method for forming an Ir film, forming a crystalline IrO2 film in a surface portion of the Ir film by performing thermal oxidation, and changing the IrO2 film to an amorphous Ir film at the time of forming a PZT film by the MOCVD method was proposed (see Japanese Laid-open Patent Publication No. 2004-253627).
Furthermore, a method for forming an amorphous IrOX film as or over an upper electrode for the purpose of preventing a PZT film from being degraded chemically or mechanically (see Japanese Laid-open Patent Publication No. 2002-261251) or the like was proposed.
For example, it is assumed that a lower electrode is formed by the use of an Ir film and that a PZT film is formed over the Ir film as a ferroelectric film by the MOCVD method.
If the MOCVD method is used, usually the PZT film is formed at a high temperature of 600° C. or higher. At this time a wafer on which the Ir film the orientation of which is controlled is formed is put first into a chamber. The temperature is raised to a target temperature in an atmosphere of argon (Ar) or oxygen (O2). Material gas is then introduced into the chamber to form the PZT film over the Ir film. From the viewpoint of productivity and polarization, it is preferable that the PZT film be preferentially oriented along the (111) plane. Therefore, it is preferable that the Ir film formed under the PZT film be preferentially oriented along the (111) plane.
However, if the temperature is raised in an atmosphere of Ar at the time of forming the PZT film by the MOCVD method, not only the (111) plane but also the (100) or (101) plane of the PZT film formed is generated. Accordingly, the (111) orientation ratio of the PZT film decreases. As a result, the electrical properties (switching electric charge amount) of a ferroelectric capacitor deteriorate.
Furthermore, if the temperature is raised in an atmosphere of O2 at the time of forming the PZT film by the MOCVD method, several problems arise. A first problem is as follows. If a PZT film is formed continuously over a plurality of wafers, variation in the (111) (or (222)) orientation ratio of a PZT film tends to occur among the plurality of wafers.
FIGS. 40 through 43 illustrate results obtained by checking the orientation of PZT films formed over a plurality of wafers. FIG. 40 illustrates the (111) integrated intensity of a PZT film in a central portion of each wafer. FIG. 41 illustrates the (111) integrated intensity of a PZT film in an edge portion of each wafer. FIG. 42 illustrates the (222) orientation ratio of a PZT film in a central portion of each wafer. FIG. 43 illustrates the (222) orientation ratio of a PZT film in an edge portion of each wafer.
The (222) orientation ratio of a PZT film is calculated by the following formula (1).(222) orientation ratio of PZT film (%)={(222) integrated intensity of PZT film}/{(100) integrated intensity of PZT film+(101) integrated intensity of PZT film+(222) integrated intensity of PZT film}×100  (1)
where (222) ((111)) integrated intensity of PZT film, (100) integrated intensity of PZT film, and (101) integrated intensity of PZT film are calculated from measurement results obtained by the use of an X-ray diffraction apparatus.
As illustrated in FIGS. 40 and 41, if the temperature is raised in an atmosphere of O2 at the time of forming a PZT film continuously over each of the plurality of wafers by the MOCVD method, there is significant variation in the (111) integrated intensity of a PZT film among the plurality of wafers, whether in a central portion or an edge portion of a wafer. As illustrated in FIGS. 42 and 43, the (222) orientation ratio of a PZT film is unstable and low. The (222) orientation ratio of a PZT film is very low especially in an edge portion of a wafer.
A second problem is that the surface of the PZT film formed tends to become rough. Extreme surface roughness or what is called cloudiness tends to appear especially in an edge portion of the wafer.
The reason for the arising of these problems is that when the temperature is raised in an atmosphere of O2, the Ir film of the lower electrode is oxidized with O2 gas. In principle, an IrOX film to which the Ir film is oxidized can be reduced again to Ir with a solvent component (such as tetrahydrofuran (THF) or butyl acetate) contained in material gas which is introduced after the temperature is raised. However, when the temperature is raised in an atmosphere of O2, abnormal oxidation of the Ir film tends to occur. When the IrOX film formed by such abnormal oxidization is reduced with the solvent component contained in the material gas, the orientation of an Ir film after the reduction degrades or the IrOX film cannot be reduced completely. As a result, the orientation of the PZT film formed over the Ir film degrades.
In the past, a method for forming a crystalline IrOX film over an Ir film was proposed. However, if a PZT film is formed by the MOCVD method, it is difficult to reduce the crystalline IrOX film uniformly at the time of forming the PZT film, or the crystalline IrOX film cannot be reduced completely to an Ir film. As a result, there is variation in the distribution of the (111) orientation of a PZT film on a wafer or among wafers or an orientation ratio higher than or equal to a certain level cannot be ensured stably.